Polycrystalline sintered bodies based on silicon nitride have, in the last few years, acquired increased importance in the field of ceramic cutting materials. These materials are made of silicon nitride and sintering additives, of mixtures of silicon nitride and other refractory compounds based on oxides, carbides and/or nitrides, and of the sialones in which Si and N atoms in the Si.sub.3 N.sub.4 are partially replaced by Al and O atoms forming solid solutions of Si--Al--O--N of different composition.
Sintered bodies produced by sintering without applying pressure from powder mixtures of Si.sub.3 N.sub.4 and sintering additives based on oxides such as MgO, Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3 in the form of presintered mixed oxides of the Spinel type are already known as cutting materials for hypereutectic Al-Si alloys (see JP-Kokai 74, 133, 803 ref. in Chem, Abstr. Vol. 84 (1976), No. 21 440 t).
Sintered bodies of the kind discussed above, however, do not have sufficient resistance to high temperatures, since the secondary phases present in the grain boundaries are predominantly vitreous and become soft at high temperatures. For this reason, they are not adequate as cutting materials under intensified conditions such as for machining iron materials.
Sintered bodies produced by hot pressing powder mixtures of Si.sub.3 N.sub.4 and oxides such as MgO, ZrO.sub.2 and Y.sub.2 O.sub.3 employing specific pressure and temperature conditions should, however, be capable of providing improved cutting materials if the amounts of admixed oxides are selected in a manner such as not to be present in the grain boundaries of the Si.sub.3 N.sub.4 matrix in Spinel form but to form small amounts of a stable, refractory oxynitride and/or silicate (See EP-A-No. 98 59 and U.S. Pat. No. 4,227,842). Besides, the formation of secondary crystalline grain boundary phases, which substantially consist of ytrrium silicon oxynitrides, must be aided by admixing other substances in small amounts such as Al.sub.2 O.sub.3, WC, or TiC (see U.S. Pat. No. 4,401,617) or can be obtained by an additional heat treatment of the sintered bodies (see DE-A-No. 30 47 255, corresponding to U.S. Pat. No. 4,264,548). If the sintered bodies are produced without applying pressure, the presence of a third substance such as Al.sub.2 O.sub.3, WC, WSi.sub.2, W and TiC is necessary in the starting powder mixture of Si.sub.3 N.sub.4 and oxide admixtures (see DE-A-No. 30 39 827 corresponding to U.S. Pat. No. 4,280,973).
Cutting materials produced by sintering with or without application of pressure from powder mixtures of Si.sub.3 N.sub.4, oxide sintering adjuvants and refractory admixtures in amounts of up to about 40% by weight of carbide (for instance, TiC) or nitride, or mixtures thereof, are known (see U.S. Pat. No. 4,388,085).
For the production of high-efficiency small cutting plates, the sialones have recently become known and can be prepared with or without application of pressure from powder mixtures of Si.sub.3 N.sub.4, AlN and Al.sub.2 O.sub.3 together with an oxide sintering adjuvant such as Y.sub.2 O.sub.3. These materials distinguish themselves by an improved resistance to high temperatures by which it is understood that the rupture modulus at 1200.degree. C. drops only slightly compared to the value measured at room temperature when after terminating the sintering operation care is taken, by a controlled gradual cooling or by a subsequent heat treatment, so that at least part of the glass in the intergranular vitreous grain boundary phases crystallizes as a ceramic phase containing, for instance, a Y-Al-garnet (see DE-C-No. 27 33 354 corresponding to U.S. Pat. No. 4,127,416).
However, it has been found that the cutting materials based on Si.sub.3 N.sub.4, according to the prior art, possess only in part the requirements relative to the properties that characterize the ideal cutting materials. Although they are in general dense, resistant to bending, oxidation and thermoshock, they are basically not sufficiently tough to withstand for long period of time without breaking, particularly without chamfering, the sudden and unexpected stresses that occur, for instance, when used as small cutting plates in continuous cutting.
Thus, the problem that presents itself is to provide polycrystalline sintered bodies based on silicon nitride, sintering additives on an oxide base and optionally other refractory admixtures based on carbide and/or nitride, which bodies distinguish themselves by combining the properties of great hardness with great fracture toughness, and that they are specially wear-resistant in order better to satisfy what is required of ceramic cutting materials for the machining of iron materials at high cutting speeds.